The present disclosure relates to semiconductor manufacturing, and more particularly, to a method for texturing a single-crystalline silicon substrate utilizing an aqueous alkaline etchant solution that includes an additive that has a higher boiling point than isopropyl alcohol.
A photovoltaic device is a device that converts the energy of incident photons to electromotive force (e.m.f.). Typical photovoltaic devices include solar cells, which are configured to convert the energy in the electromagnetic radiation from the Sun to electric energy.
In a typical solar cell, single-crystalline silicon is generally used as one of the components of the cell. In such applications, the single-crystalline silicon needs to have a non-planar surface to improve light capture. Typically, the non-planar surface has concave and convex patterns with a minute pyramid (i.e., square pyramid) shape. In such solar cells, the light reflected from one spot impinges again to another spot on the surface of the crystalline solar cell by virtue of the ‘textured’ surface, penetrating into the solar cell to be effectively absorbed in the solar cell. Although a portion of the impinging light that has not been fully absorbed, but arrives at the back face of the solar cell, is reflected back to the surface again, that portion of impinging light can be reflected again at the surface comprising steeply inclined pyramidal surfaces, thereby confining the light in the solar cell to improve absorption of light and to enhance power generation.
In conventional single-crystalline silicon solar cells, the textured structure is formed by immersing the exposed (100) face of a single-crystalline silicon wafer into a mixed solution prepared by adding 5 to 30% by volume of isopropyl alcohol into an aqueous solution of an alkaline, i.e., sodium hydroxide (NaOH) or potassium hydroxide (KOH), which may also include some added silicon. Etching in this mixed solution is performed at a temperature of from 70° C. to 95° C.
The etching rate in anisotropic etchants of the kind described above depends on the crystallographic orientation of the silicon surface being etched. The etching rate on the (111) face is significantly lower than the other crystallographic orientations. Accordingly, the (111) face with the slowest etching rate is advantageously left on the surface. Since this (111) face is inclined by about 54 degree against the (100) face, pyramidal projections constituted of the (111) face and its equivalent faces are formed. The pyramid size and density depends on the KOH or NaOH concentration, the amount of added silicon already dissolved in the bath, and additive such as isopropyl alcohol.
However, due to the high processing temperature, typically around 80° C., isopropyl alcohol, with a low boiling point of 82.5° C., volatilizes constantly. Hence, special equipment is needed to be used to capture the vapors, and the bath needs to be continuously replenished with isopropyl alcohol. Additionally, isopropyl alcohol is known to be flammable with a very low flash point. Such process and tool complexities add to the cost of solar cell fabrication.